1. Field of the Invention
This application relates broadly to improvements in automatic backwash filtering systems. More particularly, it concerns improved devices and methods for control of backwash operations in such systems.
2. Description of the Prior Art
Automatic backwash filter systems are known in the art as exemplified in U.S. Pat. Nos. 3,239,061; 4,133,766; 4,617,131; 4,764,288; 5,089,117 and 5,147,560 currently assigned to Infilco Degremont Inc., the assignee of the invention disclosed herein. The disclosures of these patents are incorporated herein by reference.
A principal use of automatic backwash filter systems is to produce potable water in water treatment plants many of which are subject to state or other governmental regulatory agencies that set standards and conditions concerning the operation of the systems and the quality of the effluent. For example, many such agencies require that, following backwash in filter beds of a potable water treatment plant, the effluent from such filter beds can not be placed back on the service line, but must be sent to waste, until their contaminant content reaches a predetermined acceptable low level, e.g., a turbidity of &lt;0.25 NTU. When this contaminant content level is reached, the filter beds can be put back in service.
The Nichols U.S. Pat. Nos. 5,089,117 and 5,147,560 respectively disclose methods and apparatus for monitoring the operation of automatic backwash filters through turbidity measurement of effluent and halt movement of backwashing from one cell (bed) to a subsequent cell until the level of turbidity of effluent from the active cell has reached a required value. However, these methods and apparatus do not address the often occurring need to keep the automatic backwash filter in service even when one or more of the many cells therein do not operate in a manner to meet the contaminant content requirement. The present invention addressed this situation and provides devices and methods that permit operation of the automatic backwash filter to continue in spite of the presence therein of one or more problem cells.